Image transformer with electronoptical image projection



Dec. 13, 1955 J. c. FRANCKEN IMAGE TRANSFORMER WITH ELECTRON-OPTICALIMAGE PROJECTION INVENTOR Jun Cure] Froncken By ent Filed Oct. 19, 1951United States INIAGE TRANSFORMER WITH ELECTRON- OPTICAL IlVlAGEPROJECTION Jan Carel Francken, Eindhoven, Netherlands, assignor toHartford National Bank and Trust Company, Hartford, Conn., as trusteeApplication October 19, 1951, Serial No. 252,071

Claims priority, application Netherlands November 6, 1950 3 Claims. (Cl.315-) This invention relates to an image transformer withelectron-optical image projection and in particular to a scanning tubein which an image formed on a photoelectric cathode is projected onto acollecting screen.

Attempts have been made to control the magnification of the imageproduced on the collecting screen in such devices in order to avoid theuse of different optical lens systems heretofore required for magnifiedreproduction of details of the image.

Magnification of the photo-cathode image electron optically can becontrolled by means of a combined magnetic and electron-acceleratingfield of rotation symmetry, using a disc of ferromagnetic material whichis arranged at right angles to the axis of symmetry of the lens andprovided with an aperture for the passage of the electron paths andwhich can be shifted in the direction of the electron paths. Shifting ofthe disc has the elfect of varying the strength of the magnetic lens andhence the image size, it being necessary to alter the energising currentin the magnet coil to retain the definition of the image on thecollecting screen, this resultant variation in the magnification being afactor of 2.

It is an object of the invention to permit control of the magnificationwithout shifting any part of the electronoptical lens in an imagetransformer.

Another object of the invention is to provide means for continuouscontrol of magnification in an image transformer.

According to the invention, an image transformer having a photoelectriccathode and using combined magnetic and electron accelerating fields,the two co-operating fields are directed substantially at right anglesto the surface of the photoelectric cathode and the electronaccelerating field is caused to diverge to a greater extent than themagnetic field. The magnification can then be controlled by varying thedivergence of the magnetic field.

In a preferred embodiment, the image transformer comprises an electrondischarge tube having a photoelectric cathode, an image collectingscreen and an energising coil for developing a magnetic field whichextends in the axial direction of the tube. The energising coil isarranged so that the magnetic field at the photocathode is substantiallyuniform. In addition, the electron discharge tube includes an electrodepositioned to apply an electric voltage with relation to thephotocathode which extends over the electron path down to a shortdistance from the photocathode. A second energising coil is alsoprovided the field of which acts on the divergence of the fielddeveloped by the first energising coil. In order to provide the desiredvariation of the electric field with relation to the magnetic field itis generally necessary for the distance between the photocathode and theacceleration anode to be bridged by the last mentioned coil. I

When the cathode image is reproduced using an electron-optical imageproducing device of the type in which a combined magnetic and electronaccelerating field is used rotation of the image is produced.Accordingly, the tube is generally rotated through a similar angle inthe opposite sense. However, the rotation varies according to thevariation in magnification and experiments have shown that. with adevice according to the invention the image rotation increasessubstantially linearly with decreasing magnification. Thus, the devicecan be constructed so that the control of the magnification is efiectedwhile at the same time position-Variation of the reproduced image byrotation of the tube is suppressed, only one adjusting member beingrequired.

It is also possible to counteract the variation of the image rotationusing three instead of two energising coils. In this case, the twoenergising coils mentioned hereinbefore are so close to the cathode thattheir fields are operative largely in the region in which the electricfield is operative, whereas the third coil is operative chiefly in aregion which is free from any electric field and hence is arrangedbetween the two first-mentioned coils and the collecting screen.

The invention will now be described with reference to the accompanyingdrawing, in which Fig. 1 shows an image transformer according to theinvention; Fig. 2 illustrates the electron paths occurring in the tube;Fig. 3 shows a television scanning tube with a control device accordingto the invention; Fig. 4 shows a tube of the type shown in Fig. 1including a further coil to compensate for the image rotation; and Fig.5 shows an alternative form of the coils shown in Fig. 1.

The image transformer shown in Fig. 1 comprises a discharge tube 1,which is constituted by a hermetically sealed glass vessel of circularcross-section, a photoelectric cathode 2 which, at the end of the narrowcylindrical part closed by a flat well, is arranged in a conventionalmanner on thebottom coated with a thin transparent conductive layer. Acollecting screen 3 is positioned at the other end of the tube which hasa larger diameter and is closed by a slightly convex wall. The screen 3is formed by applying a layer of luminescent substance to a transparentcarrier, for example a small mica plate, the latter being secured to thewall by means of a stay member 4. The inner surface of the glass wall ofthe tube 1, except for a portion in the proximity of the photocathode,is coated with an electrically conductive layer 5 which is electricallyconnected to an anode 6 extending to within a short distance of thephotocathode 2. A potential difference is applied between thephotocathode 2 and the anode 6 by means of supply conductors 7 and 8sealed in the tube wall and electrically connected respectively to theconductive layer to which the photocathode 2 is applied and the internalconductive coating 5. The voltage required is about 1000 to 6000 volts.

Cathode 2 and anode 6 together constitute a negative acceleratingelectron lens and the form of the electric field in front of the cathodeis denoted by broken lines 9 for several points of the cathode surfaceas shown in Fig. 2.

A coil 10 surrounds the tube and is arranged so that the cathode ispositioned in a substantially uniform magnetic field. A second coil 11,the field of which cooperates with that of the coil 10, is located agreater distance from the cathode and is wound on a common coil former12 with coil 10 jointly enclosed by a housing 13 of ferromagneticmaterial. The poles of the magnetic lens field are formed by flat headplates 14 to 15 of the housing 13. Energisation of the magnet coilsresults in development of a field, the variation of several lines offorce of which is shown by 16 in Fig. 2.

The image impressed on the photocathode 2 is depicted on the collectingscreen 3 by means of small electron beams formed by electrons which areemitted from an object point on the photocathode. These beams arecentrally symmetrical about their main path, i. e., the path of anelectron leaving the cathode without initial velocity.

Electrons which leave the object point with a radial or tangentialvelocity, but have no axial initial velocity, move helically about themain path which they finally intersect. In the case of a sharply definedimage these intersections of all the main paths are located in the imagesurface. More intersections may be formed along the main paths. Thedistance of these intersections from the cathode can be controlled byaltering the field intensity of the depicting coil and permits focussingseveral images of the photocathode onto the image surface. This alsoenables control of the magnification, but only stepwise since a sharplydefined image is obtained only with several adjustments of theenergising current in the depicting coil.

The electrons having an axial velocity component are responsible for theso-called chromatic aberration, an image fault which always becomesmanifest with such image projecting devices and which will be left outof consideration hereinafter.

The size of the image and the orientation with relation to the objectimage on the cathode are therefore determined by the variation of themain path. Referring to Fig. 2, the main path associated with the point13 of the photocathode 2 is designated 17. Even though magnetic lines offorce at the cathode are parallel to the axis the diverging action ofthe electrostatic forces results in the production of a component of themagnetic field strength which is at right angles to the main path. Thelatter results in a Lorenz force which brings about a rotary movement ofthe main path. The tangential velocity component of this movement bringsabout a secondary Lorenz force which is directed radially towards theaxis 19 tending to move the electron towards the axis through a distancea as in Fig. 2. As a result of the divergence of the magnetic field theangle between the direction of movement of the electron and the magneticfield lines will reverse so that the Lorenz force reverses itsdirection. The tangential velocity therefore diminishes and thenreverses its direction and the rotary movement of the main path becomesopposite to the original direction of rotation. This results in thesecondary, radially directed Lorenz force being now directed away fromthe axis. This Lorenz force directed away from the axis is furtherassisted by the centrifugal force which naturally is also directed awayfrom the axis. The main path finally strikes the collecting screen atthe point 20 which owing to the predominant influence of those twoforces is located farther away from the axis than the point 18 at thephotocathode.

It has been found that the rotation of the main path in thelast-mentioned sense is considerably greater than that which the pathperforms in the converging part of the electron lens so that the imagewhich is finally formed on the collecting screen is rotated withrelation to the cathode image. The tube may be so arranged that theviewed image is nevertheless properly oriented.

It becomes more difiicult however, when the magnification is to becontrolled. For this purpose the current in the second energising coil11 may be altered so that the divergence of'the magnetic field varies.As the current weakens the magnification increases, and vice versa.However, by varying the field of the depicting coil 10 in the oppositesense it is still possible for the image to be focussed with anymagnification. This provides constant controllability between 2.5 to75-fold magnification. In order to eleminate the disadvantage that theimage rotation is thus also varied, the device shown in Fig. 3 may beemployed.

The discharge tube shown in Fig. 3 is a televisionscanning tube, theconstruction of the part in which the image intensification occurs beingsimilar to that of the tube shown in Fig. 1. In this tube, collectingelectrode 21 is constructed as a thin insulating layer, coated with alayer of secondary emission substance which is applied to a metalsupport which is referred to as the signalling plate.

Discharge tube 22 is provided with a hollow cylindrical extension 23,for example, of glass (shown only in part) which houses a conventionalelectrode system for developing and deflecting a directional electronbeam, referred to as a scanning beam. This system usually includes anelectron gun and electrostatic and/or electromagnetic deflecting meansfor causing an electron beam generated by the gun to scan the signallingplate 21 in the wellknown manner.

The coil combination 13 is arranged at the end of the tube 22 remotefrom the collecting screen 21 and is energised by two potentiometers 24,25 which control the energising current, the current in the depictingcoil 10 being adjusted by means of potentiometer 24, whereas themagnification is controlled by the potentiometer 25. The depicting coilfocusses the image at any magnification. With a proper choice ofresistances, these adjustments may be effected simultaneously and forthis purpose rotary contact levers 26 to 27 are fitted on the commonshaft 291. A transmission ensures that rotation of the shaft 28 alsoresults in rotation of the tube 22 through an angle which is oppositeand equal to the image rotation brought about by variation of themagnification. The transmission is formed by a disc 29 which is mountedon the shaft 28, the movement of which is transmitted to an annulartrack 3% arranged to surround the tube. Adjustment is effected byturning knob 31 to alter the adjustment. The annular track also servesto support the tube, which is adapted to rotate between rollers 32.

Tr 6 current for the depicting coil It) and the control coil 11 issupplied from the direct current source 33. The coils and resistors areconnected so that upon an increase of the current in the depicting coilthe current in the control coil decreases, and vice versa.

Pig. 4 shows a device similar to that shown in Fig. l but provided witha correction coil 34 which serves to render the image rotationindependent of the variation of the magnification.

instead of using the two cylindrical coils for focussing and control ofthe magnification of the reproduced image, coils having a number ofturns extending in the opposite direction and being wound one over theother may be used as shown in Fig. 5, in which for example the coil 35acts to focus the image and the coil 36 to control the magnification. Ithas been found that by a proper choice of the number of turns in eachcoil, the variation of the ampere-turns and the variations in currentstrength this combination yields about the same results as far as thecontrol of the magnification is concerned as in the use of cylindricalcoils.

While the invention has thus been described in connection with specificembodiments and applications thereof other modifications will be readilyapparent. to those skilled in the art without departing from the spiritand scope of the invention as defined in the appended claims.

What I claim is:

1. An image transforming system comprising an electron discharge tubeincluding a planar photoelectric cathode, an anode, and a collectorelectrode, on which an image of the cathode is formed, disposed in thatorder within the tube and spaced from one another; a first coil windingsurrounding the tube in proximity to the space between the cathode andthe anode; a second coil Winding surrounding the tube in closerproximity to said cathode than said collector; means to adjustably passcurrent through said first and second coil windings to thereby vary themagnification of the image on the collector; means to apply a potentialbetween said cathode and said anode to thereby produce an electric fieldbetween the cathode and the end of the anode adjacent thereto; a thirdcoil winding disposd completely between aid first and second coilwindings and said collector and outside said electric field; and meansto adjustably pass current through said third coil winding to compensatefor image rotation produced by changes in image magnification.

2. An image transforming system as claimed in claim 1 in which a singleferromagnetic core member surrounds all three coil windings.

3. An image transforming system comprising an electron discharge tubeincluding a planar photoelectric cathode, an anode and a collectorelectrode disposed in that order within the tube and spaced apart fromone another, means to apply a potential between said cathode and anodeto thereby produce an electric field in the space therebetween, firstand second adjacent coil windings surrounding the tube and the spacebetween the cathode and anode, means to adjustably pass current throughsaid first and second windings to produce magnetic fields cooperatingwith the electric field to magnify an image and to vary thatmagnification, a third coil winding surrounding the tube and disposedbetween the first and second windings and the collector and outside thespace of the electric field, and means to adjustably pass currentthrough said third winding to compensate for image rotation produced bychanges in image magnification.

References Cited in the file of this patent UNITED STATES PATENTS

